The present invention is directed to an A.C. coupled gas-discharge display device of the multi-digit or character indicator type and more particularly to a drive circuit for driving such a display related to multiplexed operation of such display devices to provide an improved operation of the display device.
It is well-known that an electroluminescent cell can be interposed between first and second electrodes and that, upon the application of a suitable electric potential between the first and second electrodes connected to the cell, the cell will become luminescent because of the ionization which occurs within the cell. This characteristic lends itself quite readily for use in a display panel. A control circuit for driving such display is shown in U.S. Pat. No. 3,614,769, which issued Oct. 19, 1971 on the application of William E. Coleman et al. and assigned to the assignee of the present invention.
As disclosed in the Colemen et. al. patent, the application of an electric field to an electroluminescent cell causes ionization to occur within the cell. The electric field imparts energy to electrons which collide with other atoms, thus releasing other electrons. This electron multiplication process continues until breakdown occurs, at which time ignition occurs, that is, a gaseous discharge occurs within the cells, causing positive charges to be deposited on the cell walls connected to the cathode and electrons to be deposited on the cell walls connected to the anode. The charges deposited on the cell walls are trapped because of the capacitive coupling effect exerted by the cell walls. Since positive ions are attached to the cathode wall and electrons are attached to the anode wall, the wall charge will be of a polarity opposite to that of the electric field which instigated the gas discharge. In other words, the voltage contributed by the wall charge will be opposite in polarity to the applied electric field. Thus, it can be seen that after discharge occurs, the total voltage impressed on the cell will be the algebraic sums of the voltages applied to the cell terminals plus the voltage contributed by the wall charge, which after ignition is negative with respect to the applied voltage, therefore resulting in a decreased cell voltage. The gas discharge which occurs in the cell continues until the wall voltage builds up to a certain value. This value is given by the relationship V.sub.A -V.sub.W &lt;V.sub.E, where V.sub.A is the applied voltage, V.sub.W is the wall voltage, and V.sub.E is the voltage below which the cell is extinguished. In order to energize the cell again using the same magnitude of applied voltages, it is necessary to reverse the polarity of the applied voltages to the cell, thereby impressing an applied voltage across the cell which is adequate with the wall voltage left from the previous discharge, thus permitting a gas discharge to occur in the reverse direction. Since the wall charge is trapped within the cell, the wall voltage will always oppose the voltage which initiated the gas discharge.
Information is visually displayed in the display device in the form of characters, the characters being formed by a group of electroluminescent cells or segments containing an encapsulated gas. The illumination is provided by a gaseous discharge within the cell which occurs upon the application of an electric field at the cell terminals, thereby igniting the cells. Control circuits are provided for selectively energizing the electroluminescent cells, each of which is capacitively coupled between two electrodes, such as a segment electrode and a column electrode. The number of segment electrodes is determined by the number of cells per character, and the number of column electrodes is determined by the number of characters in the display device. Electrically, this takes the form of a matrix in which the columns are called segment electrodes. Each individual cell connected in a column is called a segment cell, and the segment cells in each row are connected to a character column electrode. One end of each segment electrode and each column electrode is connected to a potential source through appropriate drive transistors. The other ends of the segment and column electrodes are each connected to ground through individual driver transistors. The energization of selected segment cells in addition to the energization of a particular column electrode determines the character to be displayed. Circuit means are provided for logically controlling the drive transistors.
In order to illuminate a selected cell for display purposes, it is necessary to alternately energize the electrodes connected to the selected cells. In a multiplexing operation of each character, the cathode electrodes in each of the characters is connected to a common driver together with a selected number of characters connected to a common column driver. Each segment in the characters is connected to a segment driver. During a multiplexing operation, the display's control logic uses both the common column and segment drivers to designate which cells are to be energized. This operation occurs on a scanned "one column at a time" basis. A blanking period is required between the selection of the columns to enable the segment data to be transmitted to the control logic. Because of the large voltages involved to drive currently available displays, it has been found that the cost of the high voltage transistors necessary to handle the drive voltages increase the cost of the displays since intensity of the brightness of the electroluminescent cells is directly proportional to the frequency of switching the voltage level between a firing voltage and a non-firing voltage. These high voltage drivers have slow transition times where operating voltage levels are concerned. This operating characteristic limits the operating frequency of the drivers which directly affects the intensity of the display.
Various methods have been set forth to improve the operation of the plasma display. In the U.S. Patent to Johnson et al., U.S. Pat. No. 3,513,327, which issued on May 19, 1970, there is included the use of transformers in the driver circuits in which the secondary windings of the transformers supply the voltage to one of the elements of the display upon the application of an energizing pulse. The circuitry for controlling the operation of each of the transformers in such Johnson et al. reference is very complicated and therefore costly which, together with the cost of each of the transformers, limits the attractiveness of such a driver circuit.
It is therefore the principal object of the present invention to provide a gas display device which is brighter than those of the prior art. It is a further object of this invention to enable a gas display device to increase its luminescence utilizing low power, low voltage integrated circuit drivers. It is another object of this invention to provide a low cost gas display device having a minimum number of circuit elements that exhibits an increase in display brightness over displays presently available.